An extreme pressure additive has been typically used in lubricating fluid compositions, such as a cutting oil, a plastic working lubricant, a gear oil, a slideway oil, and grease, in order to reduce the friction and abrasion of metals and to prevent seizure. Examples of the extreme pressure additive include chlorine-containing organic compounds, such as chlorinated paraffin and chlorinated fatty acid esters, and sulfur-containing organic compounds including sulfurized fats and sulfurized olefins, such as dialkyl polysulfide; among these, dialkyl polysulfide has been widely used because it enables an extreme pressure additive to have a high sulfur content, has high solubility in a base oil, and enables a larger amount of sulfur to be added to a base oil.
Examples of dialkyl sulfide include dialkyl monosulfides and dialkyl polysulfides such as dialkyl disulfide, dialkyl trisulfide, and dialkyl tetrasulfide. A known example of such dialkyl polysulfides is a dialkyl polysulfide containing unbranched alkyl groups each having 4 to 22 carbon atoms (e.g., see Patent Literature 1). According to Patent Literature 1, this dialkyl polysulfide can be produced by the reaction of an olefin having 4 to 22 carbon atoms and unbranched terminals with sulfur and a hydrogen sulfide in the presence of a catalyst, and the produced dialkyl polysulfide is a mixture of dialkyl polysulfides each having approximately 1 to 6 sulfur atoms and generally used in the form of this produced mixture.
In dialkyl polysulfide, dialkyl monosulfide generally has a bad reactivity with metal and is unable to efficiently form a film of a metal sulfide on a metal surface. Thus, in use of a mixture of dialkyl polysulfides with the large dialkyl monosulfide content, the amount thereof in a lubricating fluid needs to be large in order to form a film of a metal sulfide. Meanwhile, among dialkyl polysulfides, a dialkyl polysulfide containing more sulfur atoms, such as dialkyl tetrasulfide, has a good reactivity with metal and therefore enables efficient formation of a film of a metal sulfide. An increase in the amount of such a dialkyl polysulfide containing more sulfur atoms, however, causes corrosion of a metal surface because of its high reactivity. Hence, in use of a mixture containing both dialkyl monosulfide and dialkyl tetrasulfide in large concentrations, the amount thereof in a lubricating fluid is limited within the narrow range that enables efficient formation of a film of a metal sulfide and that does not cause corrosion.